Unit 3A

Human Form & Function

Cells, metabolism & regulation

Regulation of fluid composition

Study Guide

Read: Human Prespectives 3A/3B

Chapter 8Complete: RQ 10-14 AYK 5-12

Structure of the kidney

Medulla

CortexCapsule

PyramidRenal artery

Renal vein

Pelvis of ureter

Collecting duct

Proximal convolute tubule

Distal convolute tubule

Loop of Henle

Peritubular capillaries

Renal corpuscle

A NEPHRON

Ureter

LS of KIDNEY

Kidneys functions

Fluid balance Salt balance Removal of wastes

(especially urea) pH balance

Interdependent

Fluid balance The kidneys play an important role in the

homeostatic regulation of body fluids (both the amount and the composition).

If we become dehydrated the kidneys can increase the reabsorption of water from the filtrate, whilst also increasing the secretion of salt. If our tissue fluids are too dilute the opposite occurs.

Water reabsorption 60-70 % of water reabsorption occurs

in the proximal convolute tubule*. The remaining 30-40 % is selectively

reabsorbed in the loop of Henle, distal convolute tubule and collecting duct, depending on our state of dehydration.

*How much water is reabsorbed at both stages depends on our state of dehydration i.e. less water is reabsorbed if our tissue fluid is dilute; more if we are dehydrated.

Urine formationThere are three stages in urine

formation: Filtration (in the renal corpuscle) Selective reabsorption (mainly in the

proximal convolute tubule – some water and salts are reabsorbed in the loop of Henle and the distal convolute tubule)

Tubular secretion (in the proximal convolute tubule and the distal convolute tubule)

EM of a glomerulus

D Gregory & D Marshall, Wellcome Images

FiltrationProcess Structure Substance Active/passive

Filtration Renal corpuscle

Filtrate

Water

Urea, Glucose, Amino acids,Vitamins, Salts (mainly sodium

& chlorine)

Passive (mass flow)

Passive

Section showing Bowman's capsule, glomerulus and tubules

Wellcome Photo Library

Bowman's capsule

Glomerulus

Tubule

Selective reabsorptionProcess Structure Substance Active/passive

Reabsorption PCT Water (60-70%)

Salts (60-70%) Glucose (100%) Amino acids (100%)Vitamins (100%)

Passive (osmosis)

All active

Loop of Henle Water (25%)Na+/Cl- (25%)

Passive (osmosis)Active

DCT Water (5%)Na+/Cl- (5%)

Passive (osmosis)Active

Collecting duct Water (5%) Passive (osmosis)

Tubular secretionProcess Structure Substance Active/

passiveTubular secretion PCT

&DCT

H+

NH4+

(ammonium)CreatinineToxins DrugsNeurotransmitters

Active

Selective water reabsorptionThe second stage of water reabsorption

is important if we become dehydrated.It can be divided into two phases (though

both are interdependent).1. The first phase involves the

reabsorption of salt under the influence of the hormone aldosterone.

2. The second phase involves the reabsorption of water under the influence of the antidiuretic hormone (ADH).

Reabsorption of salt under the influence of

aldosterone Stimulus

Decreased blood volume → reduced blood pressure

ReceptorBaroreceptors in Renal artery

TransmissionSeveral chemical messengers ending with release of aldosterone from the adrenal cortex

EffectorSodium pumps in DCT and loop of Henle

ResponseSodium reabsorbed increasing ion concentration in interstitial fluid(creates osmotic gradient)

Aldosterone stimulates sodium

pumps

Na+

Na+

Na+

Na+

HighNa+concentrationIntissuefluid

LowNa+concentrationInfiltrate

Negative feedback loopStimulus

Response Effector

Receptor

Control

centreFeedback

Decreased blood volume → reduced blood pressure

Baroreceptors in Renal artery

End-product is aldosterone from

adrenal cortex

Sodium pumps in DCT and loop of

Henle

Sodium reabsorbed

Creates osmotic gradient

Reabsorption of water under the influence of antidiuretic hormone

StimulusDecreased blood volume → reduced blood pressure → increased osmotic pressure

ReceptorOsmoreceptors in hypothalamus → (activates thirst reflex)

Transmissionnerve signal to posterior pituitary

glandADH released into bloodstream

EffectorDCT and collecting duct

ResponseIncreases permeability of above

structureswater (approx 10%) reabsorbed

ADH increases permeability of

tubuleRelativelydilutefiltrate

Relativelyconcentrated tissue fluid

Water leaves

the filtrateby osmosis

Negative feedback loop

Stimulus

Response Effector

Receptor

Control

centreFeedback

Decreased blood volume → reduced blood pressure → increased osmotic pressure Osmoreceptors in

hypothalamus

ADH from posterior pituitary gland

DCT and collecting duct

Increases permeability of DCT and collecting duct

Waterreabsorbed

Osmotic pressure maintained or reduced

Thirst reflexDrink

DeaminationDefinition - The stripping of nitrogen from

amino acid and nitrogen bases (RNA)

Deamination occurs in the liver

Amino acid → ammonia + organic compounds for respiration

Ammonia (very toxic) + CO2 → urea (H2NCONH2)

Nitrogen WastesNitrogen

compoundSource Amount Relative

Toxicity

Urea Amino Acids 21 g/day Moderate

Creatinine Muscle metabolism

1.8 g/day High

Uric acid RNA 480 mg/day Weak